This invention relates generally to methods and apparatus for wireless digital transmission and/or reception, and more particularly, to methods and apparatus for adapting wireless analog FM transceivers for digital transmission and/or reception.
At least one known digital transceiver has specialized circuitry and software (i.e., circuitry different from that of transceivers that are designed to transmit and receive only analog-FM) built into the transceiver to send and receive digital data. Transceivers having such specialized circuitry are usually more expensive than simple analog FM-only transceivers.
Some known methods for attempting to save the expense of digital communication equipment use an analog FM transmitter for transmitting and/or receiving digital information without requiring any modification to the transceiver. However, a difficulty encountered with this technique is that conventional analog FM transceivers do not pass speech audio frequencies below about 300 Hz. Speech audio frequencies this low are not necessary for maintaining intelligible speech and, furthermore, many transceivers use a low frequency audio band from 67 to 250.3 Hz for tones of a continuous tone coded squelch system (CTCSS). Hereafter, this frequency range from DC to about 300 Hz will be referred to as a “subaudible frequency band,” or simply “subaudible frequencies.” Baseband filtering in an analog FM transceiver typically rejects the subaudible frequencies from the speech audio that is transmitted and received. This rejection would normally make it very difficult or impossible to use external processing with the analog FM transceiver to produce or demodulate a digitally modulated signal (such as, for example, C4FM, which is a standard used in public safety radio systems), because the filtering removes frequency components near DC that must be retained to ensure quality in transmission such as preserving eye pattern integrity. Also, the speech audio frequencies passed by a typical analog-FM transceiver do not extend beyond about 3000 Hz because higher frequencies are not necessary for intelligible audio communications. This maximum frequency limitation places an upper limitation on the available bandwidth for the digital data. Hereafter, frequencies from about 300 Hz to 3000 Hz will be referred to as the “speech audio frequency band,” or simply “speech audio” frequencies. When referring to a transmitter portion of a transceiver or to a separate transmitter, these frequencies may be referred to as the “transmit speech audio frequency band.” When referring to a receiver portion of a transceiver or to a separate receiver, these frequencies may be referred to as the “receive speech audio frequency band.”
In addition, given the rather loose audio response tolerances in the transmit and receive paths of typical analog FM transceivers, there is likely to be a significant departure from the flat magnitude, linear phase frequency response necessary to preserve the integrity of the digital signal through the transmit/receive path. Also, even though filtering employed in a typical analog FM transceiver may be suitable for handling analog FM signals, such filtering is likely to introduce too much distortion to achieve an acceptable digital eye pattern.